Stack and tandem injection molding devices provide an advantage over single molding in that each enables the output of an injection molding machine to be at least doubled without significantly increasing its size. Stack mold configurations generally employ a stationary first platen, a movable center platen and a movable second platen. The mold cavities are conventionally located on opposing faces of the movable center platen. The movable center platen and the second movable platen reciprocate to open and close the mold cavities during a production cycle. In a stack molding apparatus, the melt runner system or the manifold system extends through the center platen in order to reach the mold cavities located on each side of the center platen via an equal path length.
Typically, multi-cavity stack molds use a single and movable sprue bar, or transfer nozzle, to provide a direct melt channel between the nozzle of the injection molding machine and its hot runner distributor, or manifold, which is mounted in the center section of the stack mold. The manifold delivers melt from the sprue bar, or transfer nozzle, into injection nozzles that are associated with each individual mold cavity. “Sprue bars” are typically thermal gated, whereas “transfer nozzles” are valve gated.
As a result of the reciprocating action of the movable platens, the sprue bar, or transfer nozzle, is continuously coupled to and decoupled from the melt source. It is therefore necessary to control the flow of the pressurized melt stream when the sprue bar, or transfer nozzle, is decoupled from the melt source so that substantially no drooling and no stringing occurs. In the case of tandem injection molding, where there are different parts in the mold that are filled in separate cycles, there is a higher potential for drooling because of the increase in cycles.
Valve gated stack injection molding apparatus, such as disclosed in U.S. Pat. Nos. 4,212,626, 4,244,909 and 5,460,510 which are incorporated herein by reference thereto, control the flow of melt between platens using valve pin members located in the melt channels of transfer nozzles. In the valve gated stack injection molding apparatus of U.S. Pat. No. 5,460,510, the upstream and downstream nozzles each have valve pins that reciprocate between retracted open positions and closed positions in which the valve pins are seated in respective gates. Diagonal vent bores are formed in the valve pins to provide a path for melt to flow back into the melt channel of the nozzle when the valve pins are in the closed position.
Tandem molds are known and they provide at least two molds that can be operated sequentially in order to mold different parts that may require different cooling cycle times. Reference is made in this regard to U.S. Pat. No. 4,400,341, U.S. Pat. No. 5,049,343 and U.S. Pat. No. 5,620,723, herein incorporated by reference, that show injection molding machines where molded parts are injected in two different mold plates that can be operated sequentially in order to adjust injection molding parameters such as the cooling time.
It is therefore an object of the present invention to provide a melt transfer device for substantially reducing the occurrence of drooling and stringing in a stack or a tandem injection molding apparatus.